Silicon On Insulator (SOI) technology is becoming increasingly important for high performance thin film transistors, solar cells, etc. SOI wafers consist of a thin useful layer of substantially single crystal silicon having a thickness that generally is less than one micron, with the layer supported on an insulating material.
Various structures and various ways of obtaining such wafers are known. Typically, used structures are formed with a thin film of silicon bonded to another silicon wafer with an oxide insulator layer in between.
Because of its rather high thickness, in particular as compared to the other parts, a major fraction of the cost of such structures has been the cost of the silicon substrate which supports the oxide layer, topped by the thin silicon layer. Thus, to lower the cost of SOI structures, the use of support substrate made of materials less expensive than silicon has been tried, in particular glass or glass-ceramics.
SOI structures using such glass-based substrates are called SiOG structures, as already mentioned. Processes for providing a SiOG structure are for example described by U.S. Pat. No. 7,176,528. Such a process is represented by FIG. 1. A source substrate 1b, generally made of silicon, is implanted with ionic species such as hydrogen. The implantation leads to the creation of a buried, weakened zone 7. Further, the source substrate 1b is bonded with a glass-based support substrate 1a and then separated by splitting the source substrate 1b to a depth corresponding to the penetration depth of the implanted species. In this way, a SiOG structure containing the original glass-based support substrate 1a and a layer 8 from the source substrate 1b, and a remaining delaminated substrate being a part of the former source substrate 1b are produced.
It is not a simple matter, however, to replace a traditional SOI support substrate with a glass-based support substrate. One potential issue with SiOG is that a glass-based support substrate 1a generally contains metals (in particular alkali metals) and other components which may be harmful to silicon or other semiconductor materials of the useful layer 8 from the source substrate 1b. Therefore, a barrier layer is generally required between the glass-based support substrate 1a and the source substrate 1b. Moreover, this barrier layer may facilitate the bonding between the silicon layer 8 and the glass-based support substrate 1a by making hydrophilic the bonding surface of the silicon layer 8. In this regard, a SiO2 layer can be used as a barrier layer to obtain hydrophilic surface conditions between the glass-based support substrate 1a and the silicon layer 8.
A native SiO2 layer can be directly formed on a silicon source substrate 1b by exposing it to the atmosphere prior to bonding. Alternatively, when anodic bonding is used, the anodic bonding process produces “in situ” a SiO2 layer between the silicon source substrate 1b and the glass-based support substrate 1a. Also, a SiO2 layer can be actively deposited or grown on the source substrate 1b prior to bonding.
U.S. Pat. No. 7,176,528 discloses another type of a barrier layer which can provided by the anodic bonding process. This barrier layer is a modified top layer of glass in the glass-based support substrate 1a, namely, a glass layer having a reduced level of ions. The disclosed anodic bonding substantially removes alkali and alkali earth glass constituents and other positive modifier ions that are harmful for silicon for a distance of about 100 nm to form a thick top layer on the glass-based substrate.
Molecular bonding is then usually performed by putting the two substrate surfaces into very close contact. Pressure is applied to the substrates by means of a mechanical piston in order to locally approach the two surfaces at a sub-nanometer scale distance. In case of hydrophilic bonding, it leads to the establishment of hydrogen bonds in between water molecules adsorbed at the two hydrophilic surfaces. With the progressive establishment of hydrogen bonds at the edges of the already bonded area, the bonded area gradually increases. A bonding wave thus propagates until it reaches the edge of at least one of the substrates. Any disturbance of the bonding wave propagation or of the bonding wave closure at the edge of a substrate may lead to the trapping of an air bubble 9. Such air bubbles 9 locally prevent the substrates to bond, which causes holes in the layer 8 separated from the source substrate 1b and bonded to the glass-based support substrate 1a, as represented in FIG. 2.
There is consequently a need for a method for bonding the two substrates that avoids the trapping of bubbles during the molecular bonding.